Genetic recombination is a multistep process involving many gene products. In order to dissect the biochemical steps involved we have chosen to focus on a key early step: strand exchange between homologous parental DNAs. To date, the ability to carry out a strand exchange between a linear duplex DNA and a homologous circular single-strand DNA is unique to recombination proteins. The product of this strand exchange reaction is a joint molecule composed of a single-strand circle joined to one end of a linear duplex. Three proteins responsible for this step have been purified: uvsx from phage T4; Rec A from E. coli; and rec 1 from U. maydis. We reported about 18 months ago the partial purification and characterization of a similar recombinase or strand-exchange protein from nuclear extracts of the human B cell lymphoblastoid line RPMI 1788. The protein had two noteworthy characteristics: (1) it did not require ATP (unlike similar to that of rec 1 but opposite to that of Rec A. Over the last 18 months we have partially purified and characterized two similar proteins from mitotic S. cerevisiae and embryos of D. melanogaster. In all respects we have examined the D. melanogaster protein is similar to the human protein. The S. cerevisiae protein, however, appears to be different from the human and fruit-fly protein in that its direction of strand displacement is similar to that of Rec A. Finally, we have investigated the possible role of these recombinases in immunoglobulin heavy-chain class switching. We have obtained joint molecules from DNAs bearing the mu switch site and either the epsilon or gamma switch sites.